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Stockholders' Equity - Basic and Diluted Shares Reconciliation (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2021
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Numerator (basic and diluted):                      
Net income $ 53.3 $ 12.2 $ 1.8 $ 21.6 $ 15.3 $ 24.5 $ 21.5 $ 22.5 $ 88.9 $ 83.8 $ 85.8
Less: Dividends paid to participating securities                 (0.6) (0.7) (0.4)
Less: Undistributed earnings available to participating securities                 (0.5) (0.4) (0.2)
Undistributed and distributed earnings available to common stockholders                 $ 87.8 $ 82.7 $ 85.2
Denominator:                      
Average number of common shares outstanding (shares)                 31,030,400 30,832,700 30,652,200
Effect of dilutive stock-based compensation (shares)                 369,900 271,500 186,100
Average number of common and potential common shares outstanding (shares)                 31,400,300 31,104,200 30,838,300